Embodiments of the present invention relate to an electrochemical sensor for detecting and monitoring analytes. More specifically, but not by way of limitation, certain embodiments of the present invention provide methods of operating an electrochemical sensor to and an electrochemical sensor for, among other things, determining pH and analyzing ion content of fluids. In other embodiments, the electrochemical sensor and methods may be used to detect and measure analytes such as hydrogen sulphide, oxygen, carbon dioxide, nitrates and/or the like.
The detection and/or measurement of analyte concentration, for example particular hydrogen ion concentration or pH, are important, in a number of research, industrial, and manufacturing processes. Merely way of example, pH measurement is important in the pharmaceutical industry, the food and beverage industry, the treatment and management of water and waste, chemical and biological research, hydrocarbon production, water monitoring and/or the like. Moreover, there has been a long felt need across numerous industries for better analyte sensing techniques, especially pH detection.
In the hydrocarbon industry, analysis operations may obtain an analysis of downhole fluids usually through wireline logging using a formation tester such as the MDT™ tool of Schlumberger Oilfield Services. However, more recently, it was suggested to analyze downhole fluids either through sensors permanently or quasi-permanently installed in a wellbore or through sensors mounted on the drillstring. The latter method, if successfully implemented, has the advantage of obtaining data while drilling, whereas the former installation could be part of a control system for wellbores and hydrocarbon production therefrom.
To obtain an estimate of the composition of downhole fluids, the MDT tools may use an optical probe to estimate the amount of hydrocarbons in the samples collected from the formation. Other sensors use resistivity measurements to discern various components of the formations fluids.
Particularly, knowledge of downhole formation (produced) water chemistry is needed to save costs and increase production at all stages of oil and gas exploration and production. Knowledge of particularly the water chemistry is important for a number of key processes of the hydrocarbon production, including:                Prediction and assessment of mineral scale and corrosion;        Strategy for oil/water separation and water re-injection;        Understanding of reservoir compartmentalization/flow units;        Characterization of water break-through;        Derivation of the water cut Rw; and        Evaluation of downhole the H2S partition the oil and or water (if used for H2S measurements).        
Some chemical species dissolved in water (including, for example, Cl− and Na+) do not change their concentration when removed to the surface either as a part of a flow through a well, or as a sample taken downhole. Consequently information about their quantities may be obtained from downhole samples and in some cases surface samples of a flow. However, the state of chemical species, such as H+ (pH=−log [concentration of H+]), CO2, or H2S may change significantly while tripping to the surface. The change occurs mainly due to a difference in temperature and pressure between downhole and surface environment. In case of sampling, this change may also happen due to degassing of a sample (seal failure), mineral precipitation in a sampling bottle, and (especially in case of H2S)—a chemical reaction with the sampling chamber. It should be stressed that pH, H2S, or CO2 are among the most critical parameters for corrosion and scale assessment. Consequently it is of considerable importance to know their downhole values precisely.
The concentration of protons or its logarithm pH can be regarded as the most critical parameter in water chemistry. It determines the rate of many important chemical reactions as well as the solubility of chemical compounds in water, and (by extension) in hydrocarbon.
Analyzing samples representative of downhole fluids is an important aspect of determining the quality and economic value of a hydrocarbon formation. Similarly, analyzing properties of liquids associated with an aquifer may be important in aquifer analysis in the hydrocarbon, water production industries and/or resource management.
Electrochemical sensors using redox active species, while having advantages over potentiometric sensors, may themselves have operability issues. For example, in the food and beverage industry, the water monitoring/management industry, the biotech industry and/or the like, it may not be desirable or even allowable in accordance with regulations to have the redox active species leech/diffuse from the electrochemical sensor. Moreover, handing of sensors comprising certain redox species may be an issue. Further, leeching/removal of the redox species from the sensor may affect performance of the sensor. In addition, it may be difficult/costly to fabricate an electrochemical sensor comprising redox species. Another issue is that electrochemical sensors using microelectrode designs may be easily fouled etc. and/or may have fabrication and/or operation issues.
The present invention provides an apparatus and method for performing electrochemical measurements. More specifically, the present invention provides a robust electrochemical sensor for accurate ion selective electrochemical measurements, including pH measurements.